if your moving all your stuff to a new 3ds than youll be forced to update since you have to be logged in nintendo and both systems need to be up to date unless there is an alternative that i dont know of

It was possible before with a bit of hex editing, but thanks to the new 10.3 update, it's not possible because it requires you to sign in to your NNID (which checks your FW for the nver value).
In simpler words, there are currently no methods to use the System Transfer app without updating both systems.
If you want to continue using homebrew, you'll need to use Ninjhax, OoThax (make sure to inject the edited save file before transferring or use a save dongle), or Ironhax (requires pre-patched Ironfall, and it is safe to transfer the game but payloads must be updated beforehand).